Detailed investigations continue on the molecular basis of the calcium and phosphate absorption system, and the effect of vitamin D thereon. These include the further characterization of the vitamin D-dependent calcium-binding protein (its binding properties, amino acid sequence, crystal structure) and the molecular and cellular basis of the diffusional and active transport systems for calcium, the molecular basis of phosphate transport. In addition, attention will be given to the certain other intestinal brush border proteins possibly involved in calcium and phosphate transport, including the alkaline phosphatase-CaATPase complex, and a 80,000-90,000 dalton protein and a 45,000 dalton protein recently identified and thought to be vitamin D responsive.